The Nobel Prize of Physics 2015 goes to Takaaki Kajita from Japan and Arthur B McDonald from Canada for their discovery entailing neutrinos. The latter which are tiny subatomic particles were proven to have mass by the duo. These findings have great implications in the world of physics: the Standard Model of particle physics is now viewed from a new angle.

McDonald and Kajita. Photo credits: Reuters.

The concept of neutrinos was first proposed by Wolfgang Pauli back in 1930. It was said that they were formed through the decay of radioactive isotopes, but not much was known about them until now. According to the Standard Model, it was thought that neutrinos had no mass and that they were neutral, with no electric charge. This theory seems to be supported by the fact that neutrinos are difficult to identify as they normally pass through matter with no apparent interaction.

Years later, they were observed in the Cowan-Reines experiment in 1956. It was found that when a neutrino would interact with a nucleus in a detector a flash of light would be picked up by photomultiplier tubes in the apparatus.

As time brought forth more sensitive detectors though, scientists started coming across discrepancies. Their calculation relating to the amount of neutrinos hitting the Earth from the Sun did not match with their observations in experiments: the latter was only a third of their number.
The “missing neutrions” predicament which was named the “solar neutrino problem” has now been answered by Kajita and McDonald.

As a matter of fact, 3 different ‘flavours’ of neutrino – the electron, muon and tau – have slightly different mass. They also interact with other particles in a variety of ways.

The different types of neutrinos and other subatomic particles. Author: Miss MJ.

Kajita showed in the 1998 Super-Kamiokande experiment that neutrinos oscillate between the flavours. He detected muon neutrinos travelling from the atmosphere above and some hitting the detector from below after moving through Earth. It was speculated that the same number of neutrinos came from both directions. However, in reality those coming straight down were more numerous than those passing through the Earth implying that muon neutrinos travelling longer had more time to oscillate into tau neutrinos that were not spotted by the detector.

McDonald thereafter confirmed these results in 2001 when he detected oscillations of neutrinos from the Sun. From his calculations, it was found that there were no missing neutrinos when taking all the flavours into consideration.

Hence, the solar neutrino problem was solved. This also indicated that the Standard Model of particle physics was not complete since the oscillations would be impossible if neutrinos had no mass of their own.

Their findings have thus proved to be invaluable to other experiments concerning neutrinos all across the globe.